The present invention relates to digital memory devices, and, more particularly, to electrically programmable non-volatile memories.
In electrically programmable non-volatile memories, high voltages of about 10 to 20V are typically used to program or erase the memories. During programming or erasing, charge is stored or removed, respectively, from the floating gate of a floating gate transistor, which is the basic element of non-volatile memories.
External power supply circuits seldom provide such a high voltage. In particular, for embedded applications where the non-volatile memories are used in integrated circuits (e.g., smart cards), such a high voltage must necessarily be produced inside the memory. In most cases, this high voltage is provided by a low voltage supply using a voltage booster circuit of the charge pump type, for example. A charge pump typically includes capacitors, transistors and/or diodes. The charge pump is a fragile element in non-volatile memories. Indeed, the capacitors in such memories cannot withstand excessively high or long voltage surges in the power supply.
A non-volatile memory may undergo a voltage surge during use. That is, the low voltage supply of the memory may reach a value that is higher than a maximum value that the memory can withstand. A surge in the power supply voltage can damage or even destroy the charge pump of a memory and thus make it unusable. To limit the risks of destruction of the charge pump by excessively high power supply voltages, voltage limiter devices may be used. Voltage limiter devices limit the value of the supply voltage provided at the output of the charge pump.
Voltage limiters are not always sufficient to protect the memory because they may not limit the voltage at all points of the charge pump. A voltage surge may thus appear at a node of the charge pump without being visible at the output of the charge pump, and thus without being taken into account by the voltage limiter device. Moreover, the response time of voltage limiter devices is sometimes too long to avoid a surge in the supply voltage.
Accordingly, even with a voltage limiter device, a memory may still be subject to a voltage surge. As a result, manufacturers sometimes find that components coming off a production line are systematically rejected during a test procedure. To determine why the components have been rejected and to find a way to remedy this problem, it is often necessary to perform a complete analysis of the production line and, potentially, of the design of the product. This analysis may be very cumbersome if the cause of the problem is unknown. Similarly, users sometimes find that certain components wear out prematurely, and they are not able to easily ascertain the cause or a way to remedy it.
One object of the present invention is to provide a circuit to detect and record a voltage surge, particularly in electrically programmable non-volatile memories.
Another object of the present invention is to provide such a circuit that mitigates the above diagnostic difficulties.
These and other objects, features, and advantages are provided by a circuit for detecting and recording a voltage surge. The circuit includes a detection circuit to detect a voltage surge having a value higher than a maximum permissible value of a supply voltage provided by a power supply. The detection circuit provides a control signal if the voltage surge is detected. A high voltage circuit is also included to produce a high programming voltage from the power supply voltage, the high voltage circuit receiving the power supply voltage at a power supply input terminal and the control signal at an enabling input terminal. The circuit further includes a memory cell comprising an enabling input terminal to receive the control signal, a high voltage input terminal to receive the high voltage, and a low voltage input terminal to receive the power supply voltage.
The circuit may therefore be used to detect the occurrence of the voltage surge and then record this event in the memory cell, which is reserved for this purpose. The contents of the memory cell may also be read and used as an indicator, if necessary. The invention is well suited for non-volatile memories because suitable memory cells for use in the circuit are already available in such memories, making its implementation simple and inexpensive. Even so, the invention may also be used for other types of integrated circuits.
The detection circuit may include a capacitor divider bridge to provide a voltage that is proportional to the power supply voltage, a voltage source to provide a reference voltage, and a comparator receiving the voltage from the capacitor divider bridge at a first input terminal and the reference voltage at a second input terminal. The capacitor divider bridge may include two series-connected capacitors. Very little power is consumed in such an embodiment, making it particularly advantageous in applications such as integrated circuits for smart cards.